tL/dtsonanci   UDhiie 

Correctioh    of 

echoes  and  reverbera 
tion  in  "the  duditonu 
University  of  Illinois 


3  3c 


. 


UNIVERSITY       OF        ILLINOIS       BULLETIN 

PUBLISHED  WEEKLY  BY  THE  UNIVERSITY 

VOL.  XIII  MAY  29,  1916 .  NO.  39 

(Entered  as  secoud-class  matter  at  the  Postoffice  at  Urbana,  III.,  under  Act  of  August  24, 1912) 


BULLETIN  No.  87 


BY 
F.  R.  WATSON 

And 

JAMES  M.  WHITE 


UNIVERSITY  OF  ILLINOIS 
ENGINEERING  EXPERIMENT  STATION 


URBANA,  ILLINOIS 


PRICE:  FIFTEEN  CENTS 

EUROPEAN  AGENT 
CHAPMAN  AND  HALL,  LTD.,  LONDON 


THE  Engineering  Experiment  Station  was  established  by  act  of 
the  Board  of  Trustees,  December  8,  1903.    It  is  the  purpose  of 
the  Station  to  carry  on  investigations  along  various  lines  of  engi- 
neering and  to  study  problems  of  importance  to  professional  engineers 
and  to  the  manufacturing,  railway,  mining,  constructional,  and  indus- 
trial interests  of  the  State. 

The  control  of  the  Engineering  Experiment  Station  is  vested  in 
the  heads  of  the  several  departments  of  the  College  of  Engineering. 
These  constitute  the  Station  Staff,  and,  with  the  Director,  determine 
the  character  of  the  investigations  to  be  undertaken.  The  work  is 
carried  on  under  the  supervision  of  the  staff,  sometimes  by  research 
fellows  as  graduate  work,  sometimes  by  members  of  the  instructional 
staff  of  the  College  of  Engineering,  but  more  frequently  by  investi- 
gators belonging  to  the  Station  corps. 

The  results  of  these  investigations  arejyiblished  in  the  form  of 
bulletins,  which  record  mostly  the  experiments  of  the  Station's  own 
staff  of  investigators.  There  will  also  be  issued  from  time  to  time,  in 
the  form  of  circulars,  compilations  giving  the  results  of  the  experi- 
ments of  engineers,  industrial  works,  technical  institutions,  and 
governmental  testing  departments. 

The  volume  and  number  at  the  top  of  the  title  page  of  the  cover 
are  merely  arbitrary  numbers  and  refer  to  the  general  publications  of 
the  University  of  Illinois;  either  above  the  title  or  below  the  seal  is 
given  the  number  of  the  Engineering  Experiment  Station  "bulletin  or 
circular  which  should  be  used  in  referring  to  these  publications. 

For  copies  of  bulletins,  circulars,  or  other  information  address  the 
Engineering  Experiment  Station,  Urbana,  Illinois. 


UNIVERSITY  OF  ILLINOIS 
ENGINEERING  EXPERIMENT  STATION 

BULLETIN  No.  87  MAY,  1(H6 

THE  CORRECTION  OF  ECHOES  AND  REVERBERATION  IN 

THE  AUDITORIUM  AT  THE  UNIVERSITY  OF 

ILLINOIS 

BY 

F.  R.  WATSON,  Associate  Professor  of  Experimental  Physics,  at  the 
University  of  Illinois,  and 

PROFESSOR  J.  M.  WHITE,  Supervising  Architect 


CONTENTS 

Page 

I.     INTRODUCTION    1 

1.  Preliminary  1 

2.  Object  of  Bulletin 2 

II.     PRELIMINARY  ACOUSTICAL  INVESTIGATION 2 

III.  MODIFICATION  OF  INTERIOR  OF  AUDITORIUM  4 

3.  Installation  of  Pipe  Organ 4 

4.  Method  of  Mounting  Hair  Felt 5 

5.  The  Decoration  and  the  Lighting  System 7 

IV.  FINAL  ACOUSTICAL  INVESTIGATION 8 

G.     Investigation  of  Echoes 10 

7.     Investigation  of  Reverberation 12 

V.     DISCUSSION  AND  CONCLUSION 14 

VI.     BIBLIOGRAPHY..  ,.16 


LIST  OF  FIGUEES 

Pag« 

1.  Floor  Plan   Showing  Interior   of   the  Auditorium   at  the   University  of 

Illinois    3 

2.  View  toward  the   Stage   Showing  the  Grill  Work  for  Free  Passage   of 

Sound  from  the  Concealed  Organ 5 

3.  Photograph  Showing  the  Mounting  of  the  Hair  Felt  in  Vertical  Strips. .       G 

4.  Photograph  Showing  the  Side  Wall  of  Fig.  2  when  Completed 7 

5.  Diagram  Showing  the  Dome  Surface  with  Wooden  Ribs  Separating  the 

Hair  Felt  in  Radial  Strips 8 

6.  Photograph  Showing  Auditorium  before  Changes  Were  Made 9 

7.  Photograph  Showing  New  Interior  of  Auditorium 10 

8.  Photograph  of  Stage  Setting  Showing  Experimental  House  with  Mega- 

phones Used  for  Testing  Echoes It 

9.  Diagram  Showing  the  Reflection  of  Sound  from  the  Unpadded  Pendentive 

in  the  Rear  Wall  12 

10.     Plan  of  Auditorium  Showing  the  Concentration  of  Sound  by  the  Walls 

under  the  Balcony  13 


THE   CORRECTION  OF  ECHOES  AND  REVER- 
BERATION IN  THE  AUDITORIUM  AT 
THE  UNIVERSITY  OF  ILLINOIS 

I.    INTRODUCTION. 

1.  PRELIMINARY. — The  work  described  in  this  bulletin  may  be 
considered  as  a  continuation  of  an  earlier  investigation  on  * '  Acoustics 
of  Auditoriums. '  '* 

Figure  1  shows  the  floor  plans  of  the  auditorium  under  investiga- 
tion. The  interior  approximates  a  sphere  cut  off  on  the  lower  surface 
by  the  sloping  floor  of  the  room.  There  is  a  balcony,  but  no  gallery. 
The  balcony  projects  12  feet  over  the  main  floor  at  the  sides  and  34 
feet  in  the  rear.  The  stage  is  built  out  into  the  room  instead  of  being 
set  back  behind  a  proscenium  arch  as  originally  designed,  the  stage 
house  having  been  omitted  to  reduce  the  cost  of  the  building. 

The  domed  ceiling  is  supported  on  four  equal  arches,  and  the  side 
walls  above  the  gallery  are  double  curved  surfaces.  The  limited  ap- 
propriation for  the  building  made  it  impossible  to  embellish  the  sur- 
faces of  the  walls  and  ceiling,  and  therefore,  they  were  left  practically 
plain,  which  increased  their  power  to  reflect  sound  and  cause  echoes. 
There  are  no  windows  in  the  room,  the  daylight  lighting  being  exclu- 
sively through  a  ceiling  light  30  feet  in  diameter  in  the  center  of  the 
dome. 

The  results  set  forth  in  the  previous  bulletin  are  briefly  as  fol- 
lows. A  systematic  investigation  of  the  acoustical  properties  of  the 
Auditorium  at  the  University  of  Illinois  was  carried  on  for  several 
years.  ' '  Cut  and  try ' '  methods  of  cure  were  avoided.  It  was  shown 
by  theory  and  experiment  that  the  usual  acoustical  faults  in  a  room  are 
due  first,  to  a  reverberation,  or  undue  prolongation  of  sound,  and  sec-  • 
ond,  to  echoes ;  both  of  these  defects  being  caused  by  the  reflection  of 
sound  from  the  walls.  Various  methods  of  cure  were  considered, — the 
effect  of  padding  and  paneling  the  walls,  the  possible  advantage  of 
installing  wiresf  and  sounding  boards, t  and  finally,  the  action  of 


'Bulletin  No.  73  of  the  Engineering  Experiment  Station,  University  of 
Illinois. 

t"  Inefficiency  of  Wires  as  a  Means  of  Curing  Defective  Acoustics  of  Audi- 
toriums. ' '  Science,  Vol.  35,  p.  833.  1912. 

i"The  Use  of  Sounding  Boards  in  an  Auditorium."  Physical  Review,  Vol. 
1  (2),  p.  241,  1913.  Also  The  Brick  Builder,  June,  1913. 


4  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

the  ventilating  system.*  The  conclusion  was  drawn  that  the  most  ef- 
fective cure  lay  in  padding  the  walls  with  materials  which  absorb 
sound. 

An  experimental  diagnosis  of  the  acoustical  properties  of  the  Audi- 
torium was  made.  This  was  done  by  tracing  the  path  pursued  by  a 
small  bundle  of  sound  when  it  was  sent  in  a  definite  direction  and  not- 
ing what  became  of  it  after  reflection.  Several  methods  of  tracing 
sound  were  tried  before  a  suitable  one  was  found.  A  ticking  watch 
backed  by  a  reflector,  or  a  metronome  enclosed  in  a  box  having  a  di- 
rected horn  gave  definite  data.  However,  a  hissing  arc  light  Avith 
a  parabolic  reflector  was  much  more  satisfactory  and  gave  conclusive 
results.  Enough  data  were  secured  in  this  way  to  show  the  general 
behavior  of  the  sound  in  the  room  and  also  to  indicate  how  the  chief 
echoes  were  set  up.f  Attempts  were  then  made  to  secure  satisfactory 
acoustics  by  hanging  curtains  and  draperies  at  critical  points  suggested 
by  the  diagnosis.  This  result  was  finally  secured  by  suspending  four 
large  pieces  of  canvas  in  the  dome. 

From  the  acoustical  standpoint,  the  Auditorium  was  then  in  a 
much  improved  condition.  The  canvas,  however,  was  very  unsightly 
and  did  not  accord  with  the  architectural  features  of  the  room.  It  was 
therefore  proposed  that  the  materials  used  to  correct  the  acoustics  be 
installed  in  such  manner  as  to  remedy  this  fault.  It  was  also  proposed 
at  this  time  to  install  a  pipe  organ,  to  decorate  the  interior  of  the  room, 
and  to  change  the  lighting  system. 

2.  Object  of  tJie  Bulletin. — The  object  of  this  bulletin  is  to  de- 
scribe the  changes  that  were  made  in  the  Auditorium  to  carry  out  the 
proposals  just  mentioned,  and  especially  to  show  how  the  acoustical 
properties  were  modified. 

II.    PRELIMINARY  ACOUSTICAL  INVESTIGATION. 

It  was  desired  that  the  materials  used  to  correct  the  acoustics  be 
installed  in  such  manner  as  to  conform  with  the  architectural  fea- 
tures of  the  Auditorium.  This  introduced  a  new  problem  since  in  the 
provisional  cure  the  canvas  sheets  in  the  dome  hung  with  very  little 
conformity  to  the  curvature  of  the  walls.  A  further  complication  ap- 
peared when  it  was  found  by  calculation  that  the  amount  of  material 
necessary  to  correct  the  reverberation  was  insufficient  to  pad  all  the 


*"Air  Currents  and  Acoustics  of  Auditoriums."     Engineering  Record.  Vol. 
67,  p.  265,  1913. 

t"  Echoes  in  an  Auditorium."     Physical  Review,  Vol.  32,  p.  231,  1911. 


BALCONY  FLOOR 
PLAN 

SCALE  >i>  v-<7" 


AUDITORIUM 
UNIVERSITY 
OF  ILLINOIS. 


FIRST 
PLAN 

SCALE  •&."«  \-a 


AUDITORfUM 

UNIVERSITY 
OF  ILLINOIS. 


FIG.  1.    FLOOR  PLANS  SHOWING  INTERIOR  OF  THE  AUDITORIUM  AT  THE  UNIVEB- 
SITY  OP  ILLINOIS  WHICH  WAS  CORRECTED  FOB  ECHOES  AND  REVERBERATION. 


6  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

walls  that  produced  echoes.  It  was  desirable  to  eliminate  the  echoes, 
but  it  was  regarded  as  risky  to  install  too  much  sound  absorbing  mate- 
rial, owing  to  the  danger  of  making  the  Auditorium  too  dead  for  sound. 

Because  of  these  difficulties  it  was  decided  to  carry  on  further  ex- 
periments and  to  secure  more  data  before  deciding  on  the  final  cure. 
Accordingly,  one  large  curved  wall  was  covered  with  strips  of  one-inch 
hair  felt,  30  inches  wide,  placed  vertically  and  30  inches  apart  so  as  to 
leave  bare  spaces  between  them.  This  arrangement  was  satisfactory 
for  several  reasons ;  it  did  not  change  the  curvature  of  the  wall ;  it  used 
only  half  the  amount  of  material,  necessary  to  cover  the  entire  wall ; 
and  because  of  diffraction  and  interference  effects,  it  was  theoretically 
more  efficient  in  breaking  up  the  reflected  sound  than  if  the  same 
material  were  spread  continuously  over  the  whole  surface.  Although 
encouraging,  the  results  were  not  so  marked  as  expected  in  diminish- 
ing the  echoes. 

On  the  basis  of  this  experiment,  plans  were  made  for  covering 
other  walls  in  a  similar  way,  except  that  the  hair  felt  was  to  be 
mounted  on  wooden  ribs  built  out  from  the  wall  surface.  Such  an 
installation  seemed  more  likely  to  break  up  the  incident  sound  than 
the  first  plan  of  mounting  the  hair  felt  snugly  against  the  wall.  The 
sound  wave  on  striking  these  outer  felt  strips  would  suffer  partial 
reflection  and  change  of  phase,  while  the  remaining  portion  of  the 
sound  would  pass  through  the  open  spaces  and  be  spread  out  by  dif- 
fraction and  reflection  from  the  walls.  The  hair  felt  strips  would  op- 
pose the  incident  and  reflected  waves,  thus  breaking  up  the  original 
sound  and  diminishing  its  intensity  and  possibility  of  producing 
echoes. 

Because  the  scaffolding  erected  for  the  use  of  the  workmen  inter- 
fered with  the  passage  of  sound  waves,  the  efficiency  of  this  method  of 
placing  the  felt  could  not  be  tested  step  by  step  as  the  material  was 
mounted.  The  test  was  deferred,  therefore,  until  the  installation  was 
completed.  In  the  meantime  the  pipe  organ  was  installed,  the  interior 
was  redecorated,  and  the  lighting  system  changed,  so  that  only  the  com- 
bined effect  of  all  these  factors  on  the  acoustics  could  be  investigated. 

III.  MODIFICATIONS  OF  THE  INTERIOR  OF  THE  AUDITORIUM. 

3.  Installation  of  the  Pipe  Organ. — The  organ  was  mounted  in  a 
unique  way  by  dividing  it  into  two  parts  and  placing  them  in  lofts 
24  feet  above  the  ends  of  the  stage  with  a  distance  of  75  feet  between 
centers.  This  arrangement  placed  the  organ  at  a  considerable  distance 


CORRECTION  OF  ECHOES  AND  REVERBERATION  7 

above  the  audience.  The  absence  of  any  vertical  surface  between  the 
lofts  and  the  audience  room  prevented  any  visible  arrangement  of  the 
organ  pipes,  but  the  necessary  free  exit  of  the  sound  was  provided 
for  by  the  construction  of  ornamental  plaster  grills  covering  the  pen- 
dentives  on  either  side  of  the  stage.  (See  Fig.  2.) 


FlG.  2.  VIEW  TOWARD  THE  STAGE  SHOWING  THE  GRILL  WORK  FOR  FREE  PAS- 
SAGE OF  SOUND  FROM  THE  CONCEALED  ORGAN.  THE  ORGAN  CONSUL  IS  SHOWN  TO  THE 

LEFT.  CARPET  is  REMOVED  FROM  STAGE  IN  PREPARATION  FOR  AN  ORCHESTRA  CONCERT 

4.  Method  of  Mounting  Hair  Felt. — The  hair  felt  was  mounted 
on  thin  furring  strips  which  were  bent  to  fit  the  curvature  of  the  sur- 
faces. The  dome  above  the  arches  and  the  double  curved  side  walls  and 
single  curved  rear  wall  above  the  balcony  were  padded  in  this  way. 
The  felt  was  mounted  in  vertical  strips  on  the  west  side  wall  as  shown 
in  Fig.  3.  Fig.  4  shows  the  wall  after  the  material  was  installed  and 
decorated. 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


PIG.  3.  PHOTOGRAPH  SHOWING  THE  MOUNTING  OF  THE  HAIR  FELT  IN  VERTICAL 
STRIPS.  THE  MATERIAL  WAS  FASTENED  TO  THIN  FURRING  STRIPS  WHICH  COULD  BE 
BENT!  TO  CONFORM  TO  THE  CURVATURE  OF  THE  SURFACE 

On  the  east  balcony  wall  the  felt  was  mounted  on  wooden  ribs  so 
that  it  stood  concentric  with  the  plaster  surface  at  a  distance  of  one 
foot.  Eighteen  inches  below  the  edge  of  the  skylight  in  the  dome  radial 
strips  of  felt  which  approached  the  wall  until  they  touched  at  the 
crown  of  the  arches,  were  mounted  on  wooden  ribs.  (Fig.  5.)  The  hair 
felt  used  was  the  Akustikos  Felt  developed  especially  for  correction  of 
acoustical  faults  by  the  H.  W.  Johns-Manville  Company  under  the 
direction  of  Professor  Sabine. 

Before  the  changes  were  made  in  the  Auditorium,  Professor  Sabine 
visited  the  building  at  the  invitation  of  President  James.  After  this 
visit,  he  wrote  to  President  James  as  follows:  "If  such  confirmation 
of  the  results  of  Professor  Watson's  investigation  is  necessary,  please 
permit  me  to  assure  you  that  you  will  obtain  an  excellent  effect  from 
following  out  his  suggestions  in  all  detail."  The  final  installation  was 


CORRECTION  OF  ECHOES  AND  REVERBERATION 


FIG.  4.     PHOTOGRAPH  SHOWING  THE  SIDE  WALL  OF  FIG.  2  WHEN  COMPLETED.  A 

REP  OF  SUITABLE  COLOR  WAS  STRETCHED  OVER  THE  ENTIRE  SURFACE  AND  DECORATED. 
IT  WAS  NECESSARY  FOR  THE  FREE  PASSAGE  OF  THE  SOUND  THAT  THE  MATERIAL  USED 
IN  DECORATING  SHOULD  NOT  CLOSE  THE  PORES  OF  THE  REP 


modified  somewhat  from  the  original  plans,  but  the  general  features 
were  maintained. 

5.  The  Decoration  and  the  Lighting  System. — The  modification 
of  the  lighting  system  involved  the  elimination  of  the  suspended  fix- 
tures. The  wall  brackets  were  retained,  but  the  main  lighting  was 
changed  to  a  semi-indirect  system  with  reflectors  above  the  arches  and 
around  the  skylight.  An  ivory  tone  was  selected  for  the  basic  color  in 
the  redecoration.  Ornamentation  was  stenciled  and  painted  on  the 
various  walls  and  surfaces  to  give  a  unified  effect.  With  the  excep- 
tion of  the  ornamental  borders  the  rep  covering  the  padded  surfaces 
was  left  its  natural  color.  The  difference  between  the  old  and  new  in- 
teriors is  shown  in  Figs.  6  and  7.  The  modifications  relieved  the  audi- 
torium of  its  cheerless,  barn-like  interior. 


10 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


FIG.  5.  DIAGRAM  SHOWING  THE  DOME  SURFACE  WITH  WOODEN  RIBS  SEPARATING 
THE  HAIR  FELT  IN  RADIAL  STRIPS.  THIS  FALSE  CEILING  ARRANGMENT  WAS  THOUGHT 
MORE  EFFECTIVE  FOR  ABSORPTION  OF  SOUND  THAN  IF  THE  MATERIAL  WERE  MOUNTED 
SNUGLY  AGAINST  THE  SURFACE 


IV.    FINAL  ACOUSTICAL  INVESTIGATION. 

The  remodeled  Auditorium  has  been  tested  under  varied  conditions 
for  music  and  speaking,  and  popular  opinion  has  pronounced  the 
acoustics  satisfactory.  A  speaker  with  a  moderate  voice  can  be  heard 
distinctly  by  auditors  in  the  most  distant  seats.  The  music  of  the  new 
pipe  organ,  according  to  experts,  is  satisfactorily  rendered.  The  room 
is  also  suited  for  orchestra  music,  though  for  this  case,  it  has  been 
found  advantageous  to  follow  the  usual  custom  of  leaving  the  wooden 
floor  of  the  stage  bare  of  carpet  so  as  to  reenforce  the  sound  from  the 
instruments. 


CORRECTION  OF  ECHOES  AND  REVERBERATION 

While  the  Auditorium  has  proved  to  be  generally  satisfactory,  a 
detailed  investigation  of  the  acoustical  effects  secured  by  the  modifica- 
tion of  the  room  was  thought  desirable.  A  request  was  made,  accord- 
ingly, that  auditors  report  any  echoes  or  acoustical  disturbances  how- 
ever slight  they  might  be.  About  a  dozen  replies  were  received,  and 
on  the  basis  of  these  and  other  considerations,  a  systematic  investiga- 
tion was  undertaken. 


FIG.  6.    PHOTOGRAPH  SHOWING  AUDITORIUM  BEFORE  CHANGES  WERE  MADE 

The  acoustical  results,  beneficial  and  otherwise,  may  be  anticipated 
by  considering  the  changes  made.  According  to  Sabine,  the  hair  felt 
installed  would  reduce  the  reverberation.  This  would  also  eliminate 
echoes  if  installed  on  certain  surfaces  in  accordance  with  the  analysis ; 
but,  since  the  amount  of  material  used  to  correct  the  reverberation 
was  insufficient  to  cover  all  the  walls,  acoustical  defects  might  still 
be  set  up  by  the  unpadded  surfaces,  especially  by  the  pendentives. 
The  pipe  organ,  by  generating  musical  sounds  that  emerged  through 
the  pendentives  in  the  dome,  might  introduce  new  acoustical  dis- 
turbances. The  openings  made  in  the  surfaces  of  two  of  the  penden- 
tives for  the  passage  of  the  organ  music  would  reduce  the  general 


12 


ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


FIG.  7.  PHOTOGRAPH  SHOWING  NEW  INTERIOR  OP  AUDITORIUM.  THE  SUSPENDED 
LIGHTING  FIXTURES  WERE  REMOVED,  THE  INTERIOR  REDECORATED,  AND  THE  REAR  WALL 
IN  THE  ALCOVE  PADDED 

reverberation  and  would  also  diminish  echoes.     The  changes  in  the 
decoration  and  in  the  lighting  system  would  produce  little  effect. 

6.  Investigation  of  Echoes. — Tests  were  made  in  several  ways  to 
determine  the  presence  of  echoes.  The  opinion  offered  by  auditors  that 
the  echoes  had  generally  disappeared  was,  of  course,  the  most  satisfac- 
tory evidence.  One  test  was  made  by  talking  through  a  megaphone 
toward  different  walls  (Fig.  8).  The  sound  was  generated  inside  a 
small  house  and  its  direction  of  propagation  controlled  by  two  mega- 
phones, one  being  pointed  toward  an  observer  and  the  other  toward  a 
wall  which  previously  gave  echoes.  No  distinct  echo  could  be  obtained 
by  speaking  simultaneously  into  the  two  megaphones.  The  ticks  of 
a  metronome  produced  very  little  additional  effect,  but  when  a  sharp 
intense  metallic  sound  was  tried,  echoes  were  obtained  from  the  un- 
padded  walls  but  only  faint  responses  from  the  padded  walls.  The 
intense  hissing  sound  of  an  arc  light  backed  by  a  parabolic  reflector 
gave  more  pronounced  results.  It  showed  that  the  padded  walls  pro- 
duced a  marked  effect  in  reducing  the  intensity  of  the  sound. 


CORRECTION  OF  ECHOES  AND  REVERBERATION 


13 


TIG.  8.     PHOTOGRAPH  OF  STAGE  SETTING  SHOWING  EXPERIMENTAL  HOUSE  WITH 
MEGAPHONES  USED  FOB  TESTING  ECHOES 


The  effect  of  the  unpadded  pendentives  in  the  rear  dome  surface  is 
shown  in  Fig.  9.  The  cone  of  incident  sound  received  by  each  pen- 
dentive  is  small  and,  after  reflection,  spreads  over  a  large  area.  It 
was  therefore  anticipated  that  little  disturbance  would  result.  This 
prediction  was  not  entirely  correct  since  the  echoes  reported  by  audi- 
tors, so  far  as  could  be  ascertained,  came  from  these  two  walls.  An 
echo  was  perceptible  when  the  speaker  faced  directly  toward  one  of 
these  pendentives  so  that  the  profile  of  his  face  was  seen  by  an 
auditor  seated  at  one  side  of  the  auditorium.  The  direct  sound  coming 
to  the  auditor  was  then  diminished  while  the  reflected  sound  was  aug- 
mented, thus  producing  an  echo. 

Other  unpadded  walls,  notably  the  side  walls  under  the  balcony, 
still  set  up  concentrations  of  sound.  Thus,  an  observer  at  A,  Fig.  10, 
can  hear  not  only  the  direct  sound  from  the  speaker,  but  also  the  por- 
tion that  is  concentrated  by  reflection  from  B.  He  does  not  hear  an 
echo  because  the  time  interval  between  the  direct  and  reflected  sounds 
is  too  short  to  enable  his  ear  to  detect  them  separately.  The  result  is 


14  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 


FIG.  9.  DIAGRAM  SHOWING  THE  REFLECTION  OF  SOUND  FROM  THE  UNPADDED 
PENDENTIVE  IN  THE  REAR  WALL.  ECHOES  SET  UP  BY  THIS  WALL  CAN  OCCASIONALLY 

BE    NOTED 

much  the  same  as  if  his  neighbor  on  the  side  toward  the  wall  were  to 
say  the  words  of  the  speaker  in  his  ear  at  the  same  time  that  he  re- 
ceived them  from  the  speaker.  The  auditor  realizes  that  something 
is  peculiar  about  the  sound  but  usually  does  not  understand  the  cause 
of  the  trouble.  An  auditor  at  C,  however,  may  get  an  echo  when  the 
speaker  faces  the  point  D. 

7.  Investigation  of  the  Reverberation. — By  means  of  Sabine's  for- 
mula and  coefficients  of  absorption*  the  time  of  reverberation  of  the 
Auditorium  was  found  and  a  calculation  was  made  to  determine  the 
amount  of  sound  absorbing  material  necessary  to  correct  the  fault. 
The  following  tabulation  shows  the  method  employed : 

•American  Architect,  1900. 


CORRECTION  OF  ECHOES  AND  REVERBERATION 


15 


SPEAKER. 


\ 


FLOOR.  PLAN 


FIG.  10.     i  LAN  OF  AUDITORIUM  SHOWING  CONCENTRATION  OF  SOUND  BY  THE 

WALLS  UNDER  THE  BALCONY 


Material  Exposed  Area 

in  sq.  meters 

Plaster  on  lath 2000 

Plaster  on  tile  510 

Wood  work 1630 

Glass    83 

Cocoa  matting  145 

Wood  seats  .  . .  2150  seats 


Average  audience 1200  people 


Volume  of  room . .  . .  12000  cubic  meters. 


Coeff.  of 
Absorption 

0.0330 

Total 
Absorption 

66.0 

0.0250 

13.0 

0.0610 

99.0 

0.0270 

2.3 

0.0200 

2.9 

0.0082 

17.7 

201 

0.44 

527 

Total        728 


16  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

Substituting  these  data  in  the  formula  t =0.1 64  V-^-a,  in  which  t  is  the 
time  of  reverberation,  V  the  volume  of  the  room  and  a  the  total  ab- 
sorbing power,  the  following  equation  for  the  empty  room  is  obtained : 

*=0.164X12000-^201=9.8  seconds. 

When  an  audience  of  1200  people  is  present, 

*=0.164Xl2000-f-728=2.7  seconds. 

This  value  isi  too  great  for  good  acoustics  and  a  reverberation  re- 
sults. To  correct  the  fault,  absorbing  material  should  be  added  until 
the  time  of  reverberation  is  reduced  to  about  1.8  seconds;  this  value 
having  been  found  satisfactory  for  halls  as  large  as  the  Auditorium 
when  used  for  both  music  and  speaking. 

The  amount  of  Akustikos  Felt  needed  to  carry  out  the  plans  al- 
ready described  was  3315  square  feet.  This  was  less  than  the  area 
necessary  for  felt  mounted  snug  against  the  wall  since  the  coefficient 
of  absorption  is  greater  when  the  felt  is  mounted  out  from  the 
wall.*  Calculations,  which  allowed  for  the  sound  absorbing  power  of 
the  felt  and  the  other  alterations  in  the  Auditorium  indicated  that 
the  time  of  reverberation  would  be  reduced  to  about  1.90  seconds  with 
1200  people  present. 

V.      DISCUSSION  AND  CONCLUSIONS. 

The  Auditorium  fulfilled  the  theory  held  many  years  ago  by  Lord 
Rayleigh*  that  a  large  room  with  hard,  non-porous  walls  and  with 
few  windows  has  a  prolonged  resonance,  and  that  the  best  chance  of 
improvement  lies  in  padding  the  walls  and  ceiling  with  sound  absorb- 
ing materials.  Thus,  the  installation  of  hair  felt  in  the  Auditorium 
reduced  the  reverberation;  the  amount  of  reduction  being  calculated 
in  advance  by  Sabine'sf  formula  and  constants  of  absorption. 

The  amount  of  hairfelt  necessary  to  correct  the  reverberation  was 
insufficient  to  cover  all  the  walls,  and  it  was  found  that  some  of  these 
unpadded  surfaces  still  produced  echoes.  This  action  was  anticipated 
in  part  from  the  general  considerations  discussed  by  Rayleigh:}:  in 
which  the  possibility  of  reflection  of  sound  was  shown  to  depend  on  the 
positions  of  the  source  and  receiver  of  sound,  and  also  upon  the  size 
and  form  of  the  wall  compared  with  the  wave  length  of  the  incident 
sound. 


*Sabine.    Architectural  Quarterly  of  Harvard   University,  p.  22,  March,  1912. 

•Theory  of  Sound,  Vol.  2,  pp.  287  and  351. 

tAmerican  Architect,  1900. 

t  Theory  of  Sound,  Vol.  2,  p.  28°. 


CORRECTION  OF  ECHOES  AND  REVERBERATION  17 

The  installation  in  an  auditorium  of  considerable  sound  absorbing 
material  eliminates  the  objectional  condition  of  satisfactory  reverbera- 
tion being  wholly  dependent  on  the  sound  absorbing  power  furnished 
by  an  audience.  This  means  that  rehearsals  without  an  audience  can 
be  conducted  satisfactorily  and  that  a  speaker  addressing  a  small 
audience  is  not  obliged  to  contend  with  a  distressing  reverberation. 

The  theoretical  advantages  in  absorbing  and  breaking  up  sound 
waves  when  hair  felt  is  mounted  out  from  a  wall  instead  of  placed 
snugly  against  the  surface  do  not  appear  to  be  so  great  as  expected. 
Observers  listened  to  sounds  reflected  from  both  types  of  surface  and 
concluded  that  a  surface  having  the  hair  felt  mounted  out  from  the 
wall  was  more  efficient.  The  conclusions,  however,  should  be  checked 
by  quantitative,  instrumental  measurements  since  the  ear  is  inaccurate 
in  its  estimation  of  the  comparative  intensities  of  different  sounds.* 
It  appears  that  the  felt  is  more  effective  when  mounted  out  from  the 
wall,  but  there  is  some  question  whether  or  not  the  advantages  se- 
cured justify  the  additional  expense  of  installation  and  the  greater 
risk  of  fire. 

The  music  of  the  pipe  organ  emerging  in  large  volume  from  the 
pendentives  in  the  dome  introduced  concentrations  of  sound  different 
from  those  set  up  when  the  source  of  sound  was  on  the  stage.  This 
made  it  desirable  to  pad  other  walls  in  addition  to  those  requiring 
padding  for  the  single  source  of  sound. 

The  effect  of  the  organ  music  confirmed  one  conclusion  set  forth 
by  Jagerf ;  namely,  that  the  strength  of  the  source  of  sound  for  good 
acoustics  should  be  inj  correct  proportion  to  the  volume  of  the  room. 
It  appears  that  the  Auditorium  is  too  small  for  loud  organ  music  since 
the  sound  in  this  case  becomes  unpleasantly  intense.  On  the  other 
hand,  it  appears  that  the  volume  is  fairly  well  suited  for  softer  organ 
music  and  for  a  weak  source  of  sound,  such  as  a  speaker  with  a  mod- 
erate voice.  In  this  connection  Jager  contends  that  an  auditorium 
is  limited  in  its  acoustical  possibilities;  that  if  a  room  is  too  large, 
it  is  impossible  to  make  it  satisfactory  for  weak  sources  of  sound.  He 
points  out  also  that  the  problem  of  correcting  faulty  acoustics  must 
include  a  consideration  of  intensity  of  sound  as  well  as  of  reverbera- 
tion; that  is,  the  variable  factors  at  command,  the  volume  and  ab- 
sorbing power  of  the  room  and  the  source  of  sound,  must  be  so  propor- 


*Kayleigh,  Scientific  Papers,  Vol.  II,  p.  132. 

\"Zur  Theorie  des  Nachhals, "  Sitzungsberichten  der  Kaiserl.  Akademie  der 
Wissenschaften  in  Wien.  Matem-naturw.  Klasse;  Bd.  CXX,  Abt.  Ila,  Mai, 
1911. 


18  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

tioned  as  to  give  not  only  a  suitable  reverberation  but  also  an  accepta- 
ble intensity  of  sound.  He  discusses  the  limitations  in  obtaining  this 
desired  result. 

Another  deduction  made  by  Jager  which  applies  rather  directly  to 
the  Auditorium  is  that  the  ratio  S/W  should  be  large  for  good  acous- 
tics, in  which  S  is  the  total  surface  of  walls,  furniture,  and  fixtures 
struck  by  the  sound  and  W  is  the  volume  of  the  interior.  Theoretically, 
this  ratio  is  smallest  for  a  sphere,  and,  since  the  Auditorium  approx- 
imates a  hemisphere,  the  excessive  reverberation  might  have  been  pre- 
dicted. 

Reverberations  and  echoes  were  corrected  simultaneously  by  in- 
stalling a  suitable  amount  of  hair  felt  on  the  walls  which  produced 
echoes.  To  locate  these  walls,  a  new  method  was  developed  in  which 
the  source  of  sound  was  an  arc  light  as  explained  earlier  in  this  bul- 
letin. 

The  investigation  showed  that  curved  walls  are  worse  acoustically 
than  plane  walls  since  they  produce  undesirable  concentrations  of 
sound  and  echoes.  It  also  appears  that  the  openings  in  the  penden- 
tives  for  the  organ  music  and  the  ventilation  openings  act  similarly 
to  open  windows  and  thus  reduce  reverberation  and  diminish  echoes. 

One  acoustical  disturbance  which  was  not  corrected  was  that  due 
to  talking  and  walking  in  the  foyer  and  on  the  stairs  immediately  out- 
side the  Auditorium.  The  sounds  of  footsteps  and  the  reverberation 
caused  by  loud  talking  and  accidental  noises  in  the  foyer  could  be 
reduced  by  covering  the  stairs  and  foyer  with  a  yielding  material, 
such  as  cork  and  by  padding  some  of  the  walls. 

It  is  apparent  from  this  discussion  that  the  means  employed  to 
correct  the  acoustics,  as  exemplified  by  this  complex  problem,  were 
based  upon  established  scientific  principles  and  this  investigation  and 
others  of  like  nature  have  served,  to  a  large  extent,  to  dispel  the  mys- 
tery surrounding  the  action  of  sound  in  auditoriums. 

BIBLIOGRAPHY  ON  ACOUSTICS  OF  AUDITORIUMS. 

(This  is  an  addition  to  the  bibliography  published  with  Bulletin 
No.  73  on  "Acoustics  of  Auditoriums.") 

Aigner,  Franz  " Experimentella  Studie  iiber  den  Nachall"  Sit- 
zungsberichte  der  Kaisl.  Akad.  der  Wissenschaften,  in  Wien, 
Math  -  Naturwiss,  Klasse.  Vol.  CXXIII,  Abt.  Ha,  July,  1914.  An 
experimental  investigation  of  the  reverberation  in  rooms  in  which 
instruments  were  used  instead  of  the  ear  to  measure  the  damping 


CORRECTION  OF  ECHOES  AND  REVERBERATION  19 

of  sound.  This  work  verified  Jager's  formula  and  also  the  em- 
pirical formula  and  absorption  coefficients  found  by  Sabine. 

Franklin,  W.  S.  ' '  Acoustics  of  the  Auditorium. ' '  Text-book  on  ' '  Light 
and  Sound."  Franklin  and  MacNutt,  pp.  282-290. 

McGinnis,  C.  S.  and  Harkins,  M.  R.  "The  Transmission  of  Sound 
Through  Porous  and  Non-porous  Materials."  Physical  Review, 
Vol.  33,  p.  128,  1911. 

Miller,  D.  €.  ' '  The  Science  of  Musical  Sounds. ' '  The  Macmillan  Co., 
1916.  A  set  of  lectures  giving  the  results  of  years  of  painstaking 
investigation  of  the  characteristics  of  tones  produced  by  the  voice 
and  by  different  musical  instruments. 

Moulton,  R.  H.  "A  New  Memorial  to  Washington."  Independent, 
July  5,  1915.  Describes  a  proposed  auditorium  designed  acousti- 
cally by  Professor  Sabine. 

Rood,  0.  N.  "On  a  Method  of  Studying  the  Reflexion  of  Sound- 
Waves."  Amer.  Jour,  of  Science,  (3),  Vol.  19,  p.  133, 1880. 

Sabine,  Wallace  C.  "Architectural  Acoustics:  The  Insulation  of 
Sound."  The  Brick  Builder,  Vol.  24,  p.  31,  February,  1915. 

Sabine,  Wallace  C.  ' '  Architectural  Acoustics. ' '  Jour.  Franklin  Inst. 
Vol.  CLXXIX,  p.  1,  January,  1915.  A  resume  of  his  investigations 
in  this  subject. 

Watson,  F.  R.  "Acoustics  of  Auditoriums."  Bulletin  No.  73,  Univer- 
sity of  Illinois  Engineering  Experiment  Station,  University  of 
Illinois  Bulletin,  Vol.  XI,  No.  29,  March,  1914.  Reprinted  in  full 
in  the  Scientific  American  Supplement,  Vol.  LXXVIII,  p.  358, 
December  5,  and  p.  380,  December  12, 1914. 

Watson,  F.  R.  "Acoustics  of  Theatres."  The  Play-Book,  Madison, 
Wisconsin,  Vol.  2,  p.  19,  August,  1914. 

Watson,  F.  R.  "Acoustics  of  Auditoriums."  Investigation  of  the 
Acoustical  Properties  of  the  Armory  at  the  University  of  Illinois. 
The  Brickbuilder,  Vol.  24,  p.  257,  October,  1915. 

Watson,  F.  R.  "Acoustics  of  Auditoriums.  Concerning  the  Theory 
of  Reverberation."  The  American  Architect,  Vol.  CVIII,  pp. 
369-374,  December  8,  1915.  A  translation  of  an  important  paper 
by  G.  Jager  on  the  theory  of  reverberation  showing  the  limitations 
of  halls  in  regard  to  acoustics. 

Watson,  F.  R.  "  An  Investigation  of  the  Transmission,  Reflection  and 
Absorption  of  Sound  by  Different  Materials."  Physical  Review, 
Vol.  7,  (2),  pp.  125-132,  January,  1916. 

Whitman,  F.  P.  "  On  the  Acoustical  Efficiency  of  a  Sounding  Board. ' ' 
Science,  Vol.  38,  p.  707,  November  14,  1913.  A  short  account  of 


20  ILLINOIS  ENGINEERING  EXPERIMENT  STATION 

experiments  made  to  determine  the  effect  of  a  sounding  board  on  the 
acoustical  qualities  of  the  chapel. 

Whitman,  F.  P.  "On  the  Acoustics  of  the  Chapel  of  Adelbert  Col- 
lege." Science,  Vol.  XLII,  p.  191,  August  6,  1915.  A  continua- 
tion of  the  previous  work  giving  description  of  tests  of  acoustics 
after  the  chapel  had  been  corrected  for  reverberation. 


PUBLICATIONS   OF  THE   ENGINEERING   EXPERIMENT   STATION 

Bulletin  No.  1.  Tests  of  Reinforced  Concrete  Beams,  by  Arthur  N.  Talbot.  1804. 
None  available. 

Circular  No.  1.     High-Speed  Tool  Steels,  by  L.  P.  Breckenridge.     1905.    None  available. 

Bulletin  No.  2.  Tests  of  High-Speed  Tool  Steels  on  Cast  Iron,  by  L.  P.  Breckenridge 
and  Henry  B.  Dirks.  1905.  None  available. 

Circular  No.  2.     Drainage  of  Earth  Roads,  by  Ira  O.  Baker.     1906.     None  available. 

Circular  No.  3.  Fuel  Tests  with  Illinois  Coal  (Compiled  from  tests  made  by  the  Tech- 
nologic Branch  of  the  U.  S.  G.  S.,  at  the  St.  Louis,  Mo.,  Fuel  Testing  Plant,  1904-1907), 
)>y  L.  P.  Breckenridge  and  Paul  Diserens.  1909.  None  arailable. 

Bulletin  No.  8.  The  Engineering  Experiment  Station  of  the  University  of  Illinois,  by 
L.  P.  Breckenridge.  1906.  None  available. 

Bulletin  No.  4.  Tests  of  Reinforced  Concrete  Beams,  Series  of  1905,  by  Arthur  N. 
Talbot.  1906.  Forty-five  cents. 

Bulletin  No.  B.  Resistance  of  Tubes  to  Collapse,  by  Albert  P.  Carman  and  M.  L.  Carr. 
1906.  None  arailable. 

Bulletin  No.  6.  Holding  Power  of  Railroad  Spikes,  by  Roy  I.  Webber.  1906.  Noni 
arailable. 

Bulletin  No.  7.  Fuel  Tests  with  Illinois  Coals,  by  L.  P.  Breckenridge,  S.  W.  Parr,  and 
Henry  B.  Dirks.  1906.  None  available. 

Bulletin  No.  8.  Tests  of  Concrete:  I,  Shear;  II,  Bond,  by  Arthur  N.  Talbot.  1906. 
None  arailable. 

Bulletin  No.  9.  An  Extension  of  the  Dewey  Decimal  System  of  Classification  Applied 
to  the  Engineering  Industries,  by  L.  P.  Breckenridge  and  G.  A.  Goodenough.  1906.  Revised 
Edition  1912.  Fifty  cents. 

Bulletin  No.  10.  Tests  of  Concrete  and  Reinforced  Concrete  Columns.  Series  of  1906 
by  Arthur  N.  Talbot.  1907.  None  available. 

Bulletin  No.  11.  The  Effect  of  Scale  on  the  Transmission  of  Heat  Through  Locomotive 
Boiler  Tubes,  by  Edward  C.  Schmidt  and  John  M.  Snodgrass.  1907.  None  available. 

Bulletin  No.  IS.  Tests  of  Reinforced  Concrete  T-Beams,  Series  of  1906,  by  Arthur  N. 
Talbot.  1907.  None  available. 

Bulletin  No.  13.  An  Extension  of  the  Dewey  Decimal  System  of  Classification  Applied 
to  Architecture  and  Building,  by  N.  Clifford  Ricker.  1907.  None  available. 

Bulletin  No.  14.  Tests  of  Reinforced  Concrete  Beams,  Series  of  1906,  by  Arthur  N. 
Talbot.  1907.  None  available. 

Bulletin  No.  15.  How  to  Burn  Illinois  Coal  Without  Smoke,  by  L.  P.  Breckenridge. 
1908.  Twenty-fire  cent*. 

Bulletin  No.  16.     A  Study  of  Roof  Trusses,  by  N.  Clifford  Ricker.     1908.     Fifteen  cents. 

Bulletin  No.  17.  The  Weathering  of  Coal,  by  S.  W.  Parr,  N.  D.  Hamilton,  and  W.  F. 
Wheeler.  1908.  None  available. 

Bulletin  No.  18.  The  Strength  of  Chain  Links,  by  G.  A.  Goodenough  and  L.  E.  Moore. 
1908.  Forty  cents. 

Bulletin  No.  19.  Comparative  Tests  of  Carbon,  Metallized  Carbon  and  Tantalum  Fila- 
ment Lamps,  by  T.  H.  Amrine.  1908.  None  available. 

Bulletin  No.  20.  Tests  of  Concrete  and  Reinforced  Concrete  Columns,  Series  of  1907, 
by  Arthur  N.  Talbot.  1908.  None  available. 

Bulletin  No.  21.  Tests  of  a  Liquid  Air  Plant,  by  C.  S.  Hudson  and  C.  M.  Garland. 
1908.  Fifteen  cents. 

Bulletin  No.  22.  Tests  of  Cast-Iron  and  Reinforced  Concrete  Culvert  Pipe,  by  Arthur 
N.  Talbot.  1908.  None  available. 

Bulletin  No.  23.  Voids,  Settlement,  and  Weight  of  Crushed  Stone,  by  Ira  O.  Baker. 
1908.  Fifteen  cents. 

*Bulletin  No.  24.  The  Modification  of  Illinois  Coal  by  Low  Temperature  Distillation, 
by  S.  W.  Parr  and  C.  K.  Francis.  1908.  Thirty  cents. 

Bulletin  No.  26.  Lighting  Country  Homes  by  Private  Electric  Plants,  by  T.  H.  Amrine, 
1908.  Twenty  cents. 

Bulletin  No.  26.  High  Steam  Pressures  in  Locomotive  Service.  A  Review  of  a  Report 
to  the  Carnegie  Institution  of  Washington,  by  W.  F.  M.  Goss,  1908.  Twenty-five  centt. 

Bulletin  No.  27.  Tests  of  Brick  Columns  and  Terra  Cotta  Block  Columns,  by  Arthur 
N.  Talbot  and  Duff  A.  Abrams.  1909.  Twenty-five  cents. 

Bulletin  Nn.  28.  A  Test  of  Three  Large  Reinforced  Concrete  Beams,  by  Arthur  N. 
Talbot.  1909.  Fifteen  cents. 

Bulletin  No.  29.  Tests  of  Reinforced  Concrete  Beams:  Resistance  to  Web  Stresses, 
Series  of  1907  and  1908,  by  Arthur  N.  Talbot.  1909.  Forty-five  cents. 

*Bulletin  No.  30.  On  the  Rate  of  Formation  of  Carbon  Monoxide  in  Gas  Producers,  by 
,T.  K.  Clement,  L.  H.  Adams,  and  C.  N.  Haskins.  1909.  Twenty-five  cents. 

*  Bulletin  No.  SI.     Fuel  Tests  with  House-heating  Boilers,  by  J.  M.   Snodgrass.      1909. 
Fifty-five  cents. 

Bulletin  No.  32.  The  Occluded  Gases  in  Coal,  by  S.  W.  Parr  and  Perry  Barker.  1909. 
Fifteen  cents. 

Bulletin  No.  33.  Tests  of  Tungsten  Lamps,  by  T.  H.  Amrine  and  A.  Guell.  1909. 
Twenty  cents. 

*  Bulletin    No.    34.      Teats    of    Two    Types    of    Tile-Roof    Furnaces  , under    a    Water-Tube 
Boiler,  by  .T.  M.  Snodgrass.      1909.      Fifteen  cents. 

Bulletin  No.  S5.  A  Study  of  Base  and  Bearing  Plates  for  Columns  and  Beams,  by 
N.  Clifford  Ricker.  1909.  Twenty  cents. 

Bulletin   JVo.    36.      The   Thermal    Conductivity    of    Fire-Clay    at    Hif?h    Temperatures,    by 
.T.  K.  Clement  and  W.  L.  Egy.      1909.     Twenty  rents. 

Bulletin  No.  37.  Unit  Coal  and  the  Composition  of  Coal  Ash,  by  S.  W.  Parr  and  W.  F. 
Wheeler.  1909.  Thirty-fire  cents. 

*Bulletin  No.  38.  The  Weathering  of  Coal,  by  S.  W.  Parr  and  W.  F.  Wheeler.  1909. 
Twenty-five  cents. 

*Bulletin  No.  39.  Tests  of  Washed  Grades  of  Illinois  Coal,  by  C.  S.  McGovney.  1909. 
Seventy-five  cents. 

Bulletin  No.  40.  \  Study  in  Heat  Transmission,  by  J.  K.  Clement  and  C.  M.  Garland. 
1910.  Ten  cents. 

*Bulletin  No.  41.     Tests  of  Timber  Beams,  by  Arthur  N.  Talbot.     1910.     Twenty  cents. 

*Bulletin  No.  42.  The  Effect  of  Keyways  on  the  Strength  of  Shafts,  by  Herbert  F. 
Moore.  1910.  Ten  cents. 


PUBLICATIONS   OP  THE   ENGINEERING   EXPERIMENT  STATION 

Bulletin  No.  48.  Freight  Train  Resistance,  by  Edward  0.  Schmidt.  1910.  Seventy- 
lire  cents. 

Bulletin  No.  44.  An  Investigation  of  Built-up  Columns  Under  Load,  by  Arthur  N. 
Talbot  and  Herbert  F.  Moore.  1911.  Thirty-five  cents. 

*Bulletin  No.  45.  The  Strength  of  Oxyacetylene  Welds  in  Steel,  by  Herbert  L.  Whitte- 
more.  1911.  Thirty-five  cents. 

*Bulletin  No.  46.  The  Spontaneous  Combustion  of  Coal,  by  S.  W.  Parr  and  F.  W. 
Kressmann.  1911.  Forty-five  cents. 

*Bulletin  No.  47.     Magnetic  Properties  of  Heusler  Alloys,   by  Edward  B.   Stephenson. 

1911.  Twenty-five  cents. 

*Bulletin  No.  48.  Resistance  to  Flow  Through  Locomotive  Water  Columns,  by  Arthur 
N.  Talbot  and  Melvin  L.  Enger.  1911.  Forty  cents. 

*Bulletin  No.  49.  Tests  of  Nickel-Steel  Riveted  Joints,  by  Arthur  N.  Talbot  and  Her- 
bert F.  Moore.  1911.  Thirty  cents. 

*Bulletin  No.  SO.    Tests  of  a  Suction  Gas  Producer,  by  C.  M.  Garland  and  A.  P.  Kratx. 

1912.  Fifty  cents. 

Bulletin  No.  51.  Street  Lighting  by  J.  M.  Bryant  and  H.  G.  Hake.  1912.  Thirh, 
fire  cents. 

*Bulletin  No.  62.  An  Investigation  of  the  Strength  of  Rolled  Zinc,  by  Herbert  F. 
Moore.  1912.  Fifteen  cents. 

*Bulletin  No.  S3.  Inductance  of  Coils,  by  Morgan  Brooks  and  H.  M.  Turner.  1912. 
Forty  cents. 

*Bulletin  No.  64.  Mechanical  Stresses  in  Transmission  Lines,  by  A.  Guell.  1912. 
Twenty  cents. 

*BuUetin  No.  66.  Starting  Currents  of  Transformers,  with  Special  Reference  to  Trans- 
formers with  Silicon  Steel  Cores,  by  Trygve  D.  Yensen.  1912.  Twenty  cents. 

*Bulletin  No.  66.  Tests  of  Columns:  An  Investigation  of  the  Value  of  Concrete  as 
Reinforcement  for  Structural  Steel  Columns,  by  Arthur  N.  Talbot  and  Arthur  R.  Lord.  1912. 
Twenty-five  cents. 

*BuUetin  No.  57.  Superheated  Steam  in  Locomotive  Service.  A  Review  of  Publication 
No.  127  of  the  Carnegie  Institution  of  Washington,  by  W.  F.  M.  Goss.  1912.  Forty  cents. 

*BuUetin  No.  58.  A  New  Analysis  of  the  Cylinder  Performance  of  Reciprocating  En- 
gines, by  J.  Paul  Clayton.  1912.  Sixty  cents. 

*Bulletin  No.  59.  The  Effect  of  Cold  Weather  Upon  Train  Resistance  and  Tonnage 
Rating,  by  Edward  0.  Schmidt  and  F.  W.  Marquis.  1912.  Twenty  cents. 

*Bullftin  No.  60.  The  Coking  of  Coal  at  Low  Temperatures,  with  a  Preliminary  Study 
of  the  By-Products,  by  S.  W.  Parr  and  H.  L.  Olin.  1912.  Twenty-five  cents. 

*Bulletin  No.  81.  Characteristics  and  Limitations  of  the  Series  Transformer,  by  A.  R. 
Anderson  and  H.  R.  Woodrow.  1913.  Twenty-five  cents. 

Bulletin  No.  68.  The  Electron  Theory  of  Magnetism,  by  Elmer  H.  Williams.  1913. 
Thirty-five  cents. 

Bulletin  No.  68.  Entropy-Temperature  and  Transmission  Diagrams  for  Air,  by  C.  R. 
Richards.  1913.  Twenty-five  cents. 

*  Bulletin  No.   64      Tests  of  Reinforced  Concrete  Buildings  Under  Load,  by  Arthur  N. 
Talbot  and  Willis  A.  Slater.     1913.     Fifty  cents. 

*Bulletin  No.  65.  The  Steam  Consumption  of  Locomotive  Engines  from  the  Indicator 
Diagrams,  by  J.  Paul  Clayton.  1913.  Forty  cents. 

Bulletin  No.  66.  The  Properties  of  Saturated  and  Superheated  Ammonia  Vapor,  by  G. 
A.  Goodenough  and  William  Earl  Mosher.  1913.  Fifty  cents. 

Bulletin  No.  67.  Reinforced  Concrete  Wall  Footings  and  Column  Footings,  by  Arthur 
N.  Talbot.  1913.  Fifty  cents. 

*Bulletin  No.  63.  Strength  of  I  Beams  in  Flexure,  by  Herbert  F.  Moore,  1913.  Twenty- 
cents. 

*Bulletin  No.  69.     Coal  Washing  in  Illinois,  by  F.  0.  Lincoln.      1913.     Fifty  cents. 

Bulletin  No.  70.  The  Mortar-Making  Qualities  of  Illinois  Sands,  by  C.  C.  Wiley.  1918. 
Twenty  cents. 

Bulletin  No.  71.  Tests  of  Bond  between  Concrete  and  Steel,  by  Duff  A.  Abrams.  1914. 
One  dollar. 

*Bulletin  No.  72.  Magnetic  and  Other  Properties  of  Electrolytic  Iron  Melted  in  Vacuo, 
by  Trygve  D.  Yensen.  1914.  Forty  cents. 

*Buttetin  No.  73.     Acoustics  of  Auditoriums,  by  F.  R.  Watson.      1914.      Twenty  cents. 

*  Bulletin   No.    74.      The  Tractive   Resistance  of   a   28-Ton   Electric   Car,   by  Harold   H. 
Dunn.     1914.     Twenty-five  cents. 

Bulletin  No.  75.  Thermal  Properties  of  Steam,  by  G.  A.  Goodenough.  1914.  Thirty- 
five  cents. 

*Bulletin  No.  76.  The  Analysis  of  Coal  with  Phenol  as  a  Solvent,  by  S.  W.  Parr  and 
H.  F.  Hadley.  1914.  Twenty-fire  cents. 

*Bulletin  No.  77.  The  Effect  of  Boron  upon  the  Magnetic  and  Other  Properties  of 
Electrolytic  Iron  Melted  in  Vacuo,  by  Trygve  D.  Yensen.  1915.  Ten  cents. 

*Bulletin  No.  78.     A   Study  of  Boiler  Losses,  by  A.  P.  Kratz.     1915.      Thirty-five  cents. 

*Bulletin  No.  79  The  Coking  of  Coal  at  Low  Temperatures  With  Special  Reference  to 
the  Properties  and  Composition  of  the  Products,  by  S.  W.  Parr  and  H.  L.  Olin.  1915. 
Twenty-five  cents. 

*Btdletin  No.  80.  Wind  Stresses  in  the  Steel  Frames  of  Office  Buildinsrs,  by  W.  M. 
Wilson  and  G.  A.  Maney.  1915.  Fifty  cents. 

*  Bulletin  No.   81.     Influence  of  Temperature  on  the   Strength   of   Concrete,    by   A.   B. 
McDaniel.     1915.      Fifteen  cents. 

*  Bulletin  No.  82.     Laboratory  Tests  of  a   Consolidation   Locomotive,  by  E.  C.   Schmidt, 
J.  M.   Snodgrass,   and  R.   B.  Keller.      1915.     Sixty-five  cents. 

*Bulletin  No.  88.  Magnetic  and  Other  Properties  of  Iron-Silicon  Alloys  Melted  in 
Vacuo,  by  Trygve  D.  Yensen.  1915.  Thirty-fire  ccntx. 

*Bulletin  No.  S4.  Tests  of  Reinforced  Concrete  Flat  Slab  Structure,  by  A.  N.  Tallin! 
and  W.  A.  Slater.  1916.  Fifty  cent*. 

*Biilletin  Nn.  85.  Strength  and  Stiffness  of  Steel  Under  Biaxial  Loading,  by  A.  J. 
Becker.  1916.  Thirty-fire  cents. 

*B\dletin  No.  86.     The  Strength  of  I-Beams  and  Girders,  by  H.  F.  Moore  and  W.  W. 

Wilson         IQIfi        Thirtti    ro-nts 


THE    UNIVERSITY  OF  ILLINOIS 

THE  STATE  UNIVERSITY" 
TJRBANA 

EDMUND  J.  JAMES,  Ph.  D.,  LL.D.,  President 

THE   UNIVERSITY  INCLUDES  THE  FOLLOWING  DEPARTMENTS: 

The  Graduate  School 

The^'College  of  Liberal  Arts  and  Sciences  (Ancient  and  Mod&s*-  Language* 
nr.d  Literatures;  History,  Economies,  Political  Science,  Sociology;  Philoso- 
phy, Psychology,  Education;  Mathematics;  Astronomy;  Geology;  Physics; 
Chemistry;  Botany,  Zoology,  Entomology;  Physiology;  Art  and  Design) 

The  College  of  Commerce  and  Business  Administration   (General   Business, 
Banking,  Insurance,  Accountancy,  Railway  Administration,  Foreign  Com- 
merce;   Courses    for    Commercial    Teachers    and    Commercial    and    Civic, 
Secretaries) 

The  College  of  Engineering  (Architecture;  Architectural,  Ceramic,  Civil,  Elec- 
trical, Mechanical,  Mining,  Municipal  and  Sanitary,  and  Railway  Engi- 
neering) 

The  College  of  Agriculture  (Agronomy;  Animal  Husbandry;  Dairy  Husbandry; 
Horticulture  and  Landscape  Gardening;  Agricultural  Extension;  Teachers' 
Course;  Household  Science) 

The  College  of  Law  (three-years'  course) 

The  School  of  Education 

The  Course  in  Journalism 

The  Courses  in  Chemistry  and  Chemical  Engineering 

The  School  of  Railway  Engineering  and  Administration 

The  School  of  Music  (four  years '  course) 

The  School  of  Library  Science  (two  years'  course) 

The  College  of  Medicine  (in  Chicago) 

The  College  of  Dentistry  (in  Chicago) 

The  School  of  Pharmacy  (in  Chicago;    Ph.G.  and  Ph.C.  courses) 

The  Summer  Session  (eight  weeks) 

Experiment  Stations  and  Scientific  Bureaus:  U.  S.  Agricultural  Experiment  Sta- 
tion; Engineering  Experiment  Station;  State  Laboratory  of  Natural  His- 
tory; State  Entomologist's  Office;  Biological  Experiment  Station  on  Illi- 
nois River;  State  Water  Survey;  State  Geological  Survey;  Mine  Rescue 
Station. 

The  library  collections  contain  (December  1,  1915)  348,607  volumes  and  83,566 
pamphlets. 

For  catalogs  and  information  address 

THE  REGISTRAR 

Urbana,  Illinois 


